Deformation In Orthogonal Micro-cutting Based On Strain Gradient Plasticity Theory

The marked increase in demand for the production of miniature components in fields of optics, electronics, medicine, bio-technology, and communications is a result of advancements in miniaturization technologies. Microcutting is becoming increasingly important for its capability of producing parts with three dimensional features in a wide range of materials, high productivity, and low power consumption. The rapid development of microcutting technology requires deep understanding of the deformation mechanics in the micro-machining.According to the macro metal cutting mechanics, the workpiece material undergoes uniform deformation behavior; the cutting performance is mainly influenced by cutting machines, cutting tool geometry and cutting conditions such as velocity, feed rate and depth of cut. Noticeable size effect, ploughing effect and minimum chip thickness are observed in the microcutting processes when the uncut chip thickness decreases down to micro scale. Macro cutting mechanics based on classic continuum mechanics can't illustrate material deformation behavior at micro scale without the intrinsic length scale. Strain gradient plasticity theory can explain the size effect of metal material in the micro scale incorporates the intrinsic length scale. To date, there has been little research on the application of strain gradient plasticity theory in the microcutting mechanics.The influence of deformation behavior at micro scale and the ratio of the feed rate to cutting edge radius on the cutting forces, surface roughness, and minimum chip thickness in the orthogonal microcutting are investigated using theoretical analysis, experimental study and FEM simulation. The deformation behavior is investigated to provide theoretical guide for predicting and controlling the cutting deformations and optimizing the cutting condition parameters.Arc primary deformation zone was proposed in the microcutting processes. The orthogonal micro-milling experiments were performed to obtain chip root samples. The chip root samples were inlaid, burnished, eroded to obtain the metallographic of the primary deformation zone which is used to validate the arc primary deformation model. The micro-hardness of the material was measured to determine the distribution of the flow stress and investigate non-uniform distribution of strain and strain gradient. The dislocations in the primary deformation zone are observed by transmission electron microscopy (TEM). The dislocation density increased with decrease of uncut chip thickness. The micro-morphology of the chip root surface was observed by scanning electron microscope (SEM), two mode of facture: opening mode (I) and sliding mode (II) coexists in the microcutting processes. With the increase of the ratio of the feed rate per tooth (uncut chip thickness) to cutting edge radius, the distribution of fracture character gradually uniform. When the ratio of the feed rate to the cutting edge radius decreased, the tensile deformation is to be nonuniform and tensile gradient increased.Introduce the frame of strain gradient plasticity theory based on the dislocation mechanics. Assuming microcutting is similar to the micro indentation processes, dislocation model of arc primary deformation zone in the microcutting was developed to calculate the effective strain gradient and thickness of arc primary zone. The propose model can really reflect the shape of the primary deformation zone and show the effect of the cutting geometry on the strain gradient.The cutting forces were proposed based on the strain gradient plasticity theory to illustrate material deformation behavior at micro scale. The cutting forces are assuming to be the sum of three components: Shear force part, ploughing force part and fracture force part is respectively developed for the deformation behavior of the material in the micro scale based on the stain gradient plasticity theory. Orthogonal cutting tests were performed to validate the proposed force model. The influence of the feed rate, cutting velocity and tool edge radius on the cutting forces is analyzed. The shear angle was calculated from the experimental data, which is a linear function for the ratio of uncut chip thickness to cutting edge radius. The cutting force empirical formula of exponential format was found.The chips in the microcutting processes were corrected to analyze the chip morphology and the law of the chip compress ratio. The minimum chip thickness is determined based on the strain gradient plasticity theory to reflect the material deformation behavior at micro-scale. The surface roughness prediction model was proposed considering the minimum chip thickness effect. The appropriate ratio of uncut chip thickness to cutting edge radius was calculated according to the prediction model, which can be used to design the various cutting edge tools. A Taylor-based non-local theory of plasticity was chosen as the basis for the constitutive model which represents the material behavior under highly localized inhomogeneous deformation in microcutting. The micro slot milling experiments were performed to obtain the cutting force data and chip thickness. The inverse identification technology was used to obtain the material coefficient. A coupled thermo-mechanical finite element model formulation incorporating strain gradient plasticity is developed to simulate orthogonal microcutting processes. The thermo-mechanical model is experimentally validated. The distribution of the effective flow stress and strain in the primary deformation zone was determined and the effect of the ratio of the uncut chip thickness to cutting edge radius was analyzed on the material deformation mechanics.